Taking a look at the chord tones within each chord will give you some insight as to why the few notes played in this first solo - namely E, G and B - work so well. The kickoff chord, Em7, is made up of all three of the aforementioned notes while C#m7 has an E and a B and welcomes the sound of a b5, which is what the G is over that chord. As for the Cmaj7 you'll find E, G and B in the chord for sure as they are the 3, 5 and 7; while Am7 has an E and G (5 and b7) and could easily have B serve as an extended 9th. You'll just have to watch yourself a bit in the final sixth bar, but you can play it safe and vibe on a B and still sound great.